1. Field of the Invention
The present invention relates to a superconducting transistor, and more particularly, to a superconducting transistor in which each emitter, base and collector layer are formed by the same high temperature superconductor.
2. Description of the Related Art
Recently, an oxide high temperature superconductor having a critical temperature (Tc) higher than the boiling point of nitrogen i.e. 77 [K], has been found and will be utilized in the field of electronics.
Although the well known Josephson junction used in the field of the electronics has the advantages of a high speed and low electric power consumption, this Josephson junction element is a two terminal element and thus can not be realized merely by circuitry.
Therefore, if a three terminal element using a superconducting phenomena; i.e., an element wherein a transistor operation is carried out, can be obtained, various functions can be preferably realized. If the three terminal element can be realized while using a high temperature superconductor, technologies in a number of fields would be advanced.
FIG. 1 is a cross-sectional side view of a conventional superconducting base transistor previously developed by the present inventors.
In FIG. 1, an n.sup.+ type InGaAs collector layer 22 having a thickness of 50 nm and an impurity concentration of 2.times.10.sup.19 /cm.sup.3 is provided on a semi-insulating InP substrate 21. Further, provided on the collector layer 22, in the following order, are an n type InGaAs collector layer 23 having a thickness of 50 nm and an impurity concentration of 5.times.10.sup.17 /cm.sup.3 ; an In Al.sub.0.14 Ga.sub.0.86)As collector layer 24 having a thickness of 100 nm; an n type InGaAs collector 25 having a thickness of 10 nm and an impurity concentration of 5 .times.3 an n.sup.+ type InGaAs collector layer 26 having a thickness of 30 nm and an impurity concentration of 2 .times.10.sup.19 /cm.sup.3 ; an Nb superconducting base layer 27 having a thickness of 100 nm and formed by a magnetron sputtering process; an aluminum oxide (AlOx) tunneling barrier layer 28 having a thickness of 30 .ANG.; and, an Nb emitter layer 29 having a thickness of 100 nm and formed by a magnetron sputtering process. Between the layers 27 and 28, an aluminum layer having a thickness of 30 .ANG. is usually formed.
Further, an Nb collector contact layer 30 is provided on the collector layer 22, and an Nb emitter electrode 32, Nb base electrode 33 and Nb collector electrode 34 are connected to the Nb emitter layer 29, the superconducting base layer 27 and the Nb collector contact layer 30 respectively. Numeral 31 represents an insulating film of a silicon dioxide.
Further, non-doped InGaAs spacer layers are provided between the collector layers 23 and 24 and between the collector layers 24 and 25, although not shown in FIG. 1.
As shown in FIG. 1, in the superconducting base transistor, the collector is formed by a heterojunction of InGaAs/InAlGaAs.
When a voltage is supplied to cause a flow of current between the emitter layer 29 and the base layer 27, quasi-particles are injected into the base layer 27. A barrier having a height of the energy gap of a superconductor exists between the base layer 27 and the collector, and the injected quasi-particles pass through the barrier and flow into a collector. Superconducting leakage current passing through the collector can be stopped at the barrier.
Namely, the collector current is controlled by a current passing from the emitter to the base.
In the above-explained prior superconducting base transistor, a very small barrier must be reproducibly formed, this barrier having the same level as a superconducting gap existing between a collector consisting of a plurality of semiconductor layers and a superconducting base layer.
Generally, the interface between a semiconductor and a superconductor is unstable and an electrical barrier larger than the superconducting gap is easily formed. Consequently, due to the provision that the number of quasi-particles penetrating to the collector becomes small, a current gain can not be obtained. Further, it is difficult to establish contact between a superconducting layer and a semiconductor at a barrier height of an energy gap of a usual superconductor, e.g., about 1.5 meV.